The jumping display, known as an independent seconds or independent minutes display, or any other time interval, is always a complex complication to make, since known embodiments cause a relatively abrupt jump, which results in shocks inside the mechanism, which are transmitted as far as the escapement, which is not ideal for the proper working and longevity of the watch.
These known embodiments generally combine a mechanism using cooperation between a star wheel and a flirt on the one hand, and a constant force device on the other hand, to moderate the effects of any shock on the movement.
Thus Swiss Patent No. CH 47 297 in the name of Pellaton-Schild disclosed, in 1907, a independent jumping seconds mechanism, with two contrate gears connected to each other by a spiral spring, one of which drives the escape pinion, which is coaxial and secured to a star wheel comprising projecting teeth. Via a gear train, the going barrel drives a pinion carrying a flirt, which rests each time on one tooth of the star wheel, and the relative pivoting thereof causes a position in which the flirt escapes from the tooth, in a jump, and completes one revolution before returning to abut on another tooth of the star wheel.
There is known a European Patent Application No. 1 319 997 in the name of Richemont International SA which discloses a tourbillon mechanism incorporating a constant force device.
This tourbillon mechanism includes a pivoting cage driven by a drive wheel. The tourbillon cage is coaxial to a pivoting balance provided with a balance spring and with a fixed fourth wheel and carries, at three distinct off-centre positions, an off-centre escape wheel, a first pallet lever and a stop wheel which meshes with said fixed fourth wheel. The escape wheel cooperates with said first pallet lever provided with two pallet stones. Coaxial to the escape wheel, a constant force device includes an escape spring, secured at a first end to the escape wheel, and at a second end to a force compensating disc. The disc rotates integrally with a ring for winding the escape spring and with a winding ring pinion associated therewith which cooperates with the fixed fourth wheel.
Coaxially secured to the escape wheel, a substantially triangular Reuleux cam cooperates with a fork comprised in a second pallet lever coaxial to the balance, which pivots about the axis of the tourbillon cage. This second pallet lever comprises a cam arranged to cooperate with a fork and a dart carried by the first pallet lever. The second pallet lever includes two pallet stones arranged to cooperate with radially projecting teeth comprised in the stop wheel.
The balance is driven under the effect of the prestress of the escape wheel and is returned by the balance spring. Depending on the number of teeth of the escape wheel, the balance makes a certain number of vibrations, for example five with a wheel having fifteen teeth, before the stop wheel and tourbillon cage are released by the Reuleux cam and the second pallet lever is centred on the balance. Depending upon the number of teeth it has, the stop wheel achieves a given angular travel, for example 90°, before being stopped again by one of the pallet stones of the second pallet lever centred on the balance. Since the stop wheel and the stop pinion are carried by the tourbillon cage and since the stop pinion meshes with the fixed fourth wheel, the pivoting of the stop wheel causes the tourbillon cage to pivot. Consequently, this pivoting also causes the winding ring, which is fixed to the cage and also meshes with the fourth wheel, to pivot, tensioning the escape spring again, since the escape wheel is then locked by the first pallet lever. The escape spring is periodically tensioned again by the repetition of this cycle. The spring thus accumulates enough energy to deliver sufficient torque to maintain the oscillations of the balance.
The object of this compensation mechanism is to deliver a constant torque.
The first end of the escape spring is secured to a first pin integral with a first escape spring collet connected to the escape wheel. The second end of the escape spring is fixed to a second pin secured to a second mobile escape spring collet.
The escape spring, once taut, exerts a force on the first pin, and therefore exerts a torque on a first arm of the force compensating disc comprised in said constant force device. The latter includes first and second arms, arranged to abut respectively on the first and second pin, and the bearing surfaces of these arms are aligned with each other but in an off-centre direction relative to the axis of the escape wheel. The second arm abuts on the second pin and transmits the torque to the fixed spring collet and to the escape wheel. Due to the off-centre arrangement of the direction of the two arms, the lever arm of the stresses exerted by and on the pins varies according to the angular position of the force compensating disc, despite the lost tension of the escape spring while the gear train assembly is stopped from the spring to the stop wheel.
During each vibration of the balance, the escape wheel is released from the first pallet lever and pivots through a certain angle, under the action of the escape spring, just like the first spring collet and the force compensating disc, whereas the winding ring and stop wheel are locked. On each fifth vibration, the stop wheel and the tourbillon cage are released.
This EP Patent No. 1 319 997 therefore discloses a perfectly functional system, which provides a seconds display in a tourbillon cage, but which remains very complex, and requires a constant force device further complicated by a compensating mechanism. It has to comprise two springs, two pallet levers, a cam fork device and not only has a high production cost due to the number and complexity of the components, but is relatively fragile and difficult to adjust properly.
EP Patent Application No. 1 772 783 in the name of MONTRES BREGUET SA discloses a watch movement including a constant force device, and an independent minute display on the centre wheel, which is capable of driving a tourbillon in a satisfactory manner with the constant force device. It includes a third wheel set, which completes one revolution in several minutes, and which forms the input device of a constant force device. The output element of this balance spring device is formed by a second third wheel which meshes with the fourth pinion, which is integral with a tourbillon cage. This second third wheel is integral with a star wheel, which periodically, in this case once per minute, releases a stop train meshed with the input third wheel set, which cooperates with the centre wheel, which therefore jumps once per minute. This mechanism minimises the transmission of shocks between the constant force device and the escapement.
EP Patent Application No. 1 528 443 A1 in the name of JOURNE, which discloses an independent constant force device, is also known. An energy storage spring tends to pivot a lever. A pinion of a first fourth wheel of the movement meshes with an intermediate wheel pivotally mounted on said lever. This intermediate wheel meshes with the pinion of a second fourth wheel, the arbour of which is secured to a tourbillon escapement. The lever carries a finger, which is arranged to cooperate with a ratchet toothing of a stop wheel which meshes with the first fourth wheel. When the finger is in mesh with a radial flank of the ratchet, the gear train is stopped and there is no transmission of force between the first fourth wheel and the intermediate wheel. During this stopped period, which lasts for one second, the spring torque is released and causes the lever to rotate until the finger is released from the ratchet. The second fourth wheel is controlled by the escapement, and only rotates when the said escapement is moved by the balance. The spring is wound by the movement of the lever in the opposite direction, said spring exerting a lower torque on the balance than that exerted by the barrel spring on the lever when the stop wheel is released. This device allows the winding/letting down cycle to be adapted by choosing the number of teeth in the stop wheel. The working of the device requires the presence of the energy storage spring.
In each case, the constant force device provides an advantage, which is to ensure a relatively constant drive torque for the escapement, but which necessarily involves a significant space requirement and high costs.